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Research and Projects

PenCHORD: Modelling the optimal size of Derriford Ambulatory Care Unit (ACU)

Who is involved?

Theme: PenCHORD
Status: Complete


The Ambulatory Care Unit (ACU) at Derriford Hospital, Plymouth, treats patients who are referred from the acute GPs and the Emergency Department, who can be treated as outpatients and sent home the same day, thereby avoiding unnecessary admissions. The ACU sees patients with a range of conditions, including anaemia, some respiratory conditions, chest pain, abdominal pain, and cellulitis. The unit also undertakes transfusions and runs a number of specialty clinics.

The ACU currently sees and treats around 30-35 patients per day, but it is clear that more patients could be seen by the ACU if it had additional capacity, thereby relieving pressures on the Emergency Department and avoiding unnecessary hospital admissions.

Plymouth Hospitals NHS Trust (PHNT) was looking to move the ACU into a new space in their estate by summer 2017 to allow for the ACU to increase its capacity to meet the demand. However, it was unclear what the new capacity should be in order to ensure that the ACU could see and treat all patients referred to the hospital who could be seen in an ambulatory capacity.

PHNT asked PenCHORD to undertake some modelling work to ascertain the capacity of the ACU needed. This evidence would be used directly by the PHNT Executive Team to determine the size of the new ACU.

Project aims

This project aimed to provide modelling evidence of the optimal size of the new Ambulatory Care Unit (ACU) at Derriford Hospital, such that it could see all patients who could be seen and treated in an ambulatory capacity on the day they arrive and before close of service at 8pm. 

Project activity

Derriford Hospital provided 12 months of anonymised data on referrals to the ACU, including the type of treatment and time spent by patients at each stage of the treatment process.

PenCHORD created a Discrete Event Simulation model that captured the variability in demand for ACU appointments and the time spent in ACU. They then adjusted the capacity of the ACU to assess how increasing the capacity could help to see more patients.

The model clearly showed that the key constraint within the ACU was not physical capacity, but the time the unit is open. Currently the ACU is open Monday to Friday, 8am to 8pm. However, the additional potential activity the ACU wanted to take on arrives at a relatively consistent rate over a 24 hour period, seven days a week. The model showed that, although minor gains could be made from increasing physical capacity of the ACU, far more significant gains could be achieved by extending the ACU’s opening hours. 

Specifically, the model predicted that by extending the opening hours by four hours per day, the ACU could double the number of people seen and treated in the unit. In addition, opening at the weekends would allow them to treat patients who would have otherwise been admitted to a ward, and would only require around 50% of the resourcing needed during the week.

Impact and next steps

PenCHORD presented the modelling data and recommendations to stakeholders at Derriford Hospital. These results were then used as the central evidence to inform a £1million business case presented to the hospital’s Executive Board, which requested funding to support an ACU with extended opening hours that is open seven days a week. The business case was approved, and a project group established to take the changes forward.

View more PenCHORD projects


Plymouth Hospitals NHS Trust
Charlotte Edwards - Service Improvement Lead / Project Manager
Ryan Hunneman - Service Improvement Lead and PenCLAHRC Health Service Modelling Associate
Anna Jones - Ambulatory Care Unit Manager and Advanced Nurse Practitioner